1. Field of the Invention
This invention relates to a light measuring device of a photographic camera which is shiftable from one light measuring mode to another.
2. Description of the Prior Art
Heretofore, a light measuring method called TTL light measuring method in which the brightness or light quantity of an object to be photographed is measured by allowing a light flux coming from the object through a photo-taking lens to be incident upon a photo-electric transducer element has generally been practiced in varied modes including an averaging light measurement mode, a center emphasized averaging light measurement mode, a spot light measurement mode, etc. In addition to these modes, there has been known another light measuring method in which two different light fluxes are used to permit changeover between an averaging light measurement mode and a spot light measurement mode. This averaging/spot change-over light measurement method gives to a camera an advantage that a desired exposure value can be obtained by selecting either the feature of averaging light measurement or that of spot light measurement. This is convenient for photographing. However, many of the cameras of the type using this light measuring method use two discrete light sensitive elements or photo-electric elements for averaging light measurement and for spot light measurement separately from each other. For example, the photo-electric light sensitive element for averaging light measurement is disposed within a penta-prism viewfinder optical system and is arranged to perform an averaging or center-emphasized averaging light measuring action on the image of an object formed on a reticle; meanwhile, the photo-electric light sensitive element for spot light measurement is arranged through some spectral means to measure mainly the light of the middle part of the image; and an adequate degree of exposure is obtained through computation by the averaging or spot light measurement with the output of one of the two photo-electric light sensitive elements selected.
With the conventional light measuring devices arranged as described above, they have had the following drawbacks: (1) Two light sensitive elements (SPC) must be used. (2) The two light sensitive elements must be disposed within a limited space of the camera.
Meanwhile, in the case of the conventional devices of the type permitting selection between spot light measurement and averaging light measurement with use of a single light sensitive element, the selection is performed by means of a mechanical moving member. The devices of this type thus have had the following drawbacks: (3) The arrangement to mechanically perform selection results in a complex mechanism which is disadvantageous in terms of cost and space. (4) Photometric sensitivity distribution is limited by a light measuring optical system and this allows no latitude for design.